Zinc- and zinc alloy-coated steel sheets includes a galvanizd steel sheet, an electro-galvanized steel sheet, a zinc alloy electroplated steel sheet, a galvannealed steel sheet, etc.
The zinc- and zinc alloy-coated steel sheets have an excellent ability to protect the steel sheet themselves due to the galvanic behavior of a zinc coating layer.
In particular, a content of iron (Fe) in the galvannealed steel sheet (hereinafter, referred to as “GA steel sheet”) is varied according to the alloying degree, but the GA steel sheet has a more base standard electrode potential than Fe (−0.44 V) when Fe in the GA steel sheet is usually present in a content of 9 to 12% by weight. In this case, when a zinc coating layer is formed on the GA steel sheet, the GA steel sheet has an excellent ability to protect steel sheets the to the galvanic behavior of the zinc coating layer, and it is inexpensive and easy to be manufactured. As a result, the GA steel sheet is mainly used as a steel sheet for automobiles.
However, the zinc- and zinc alloy-coated steel sheet has a disadvantage that the coating layer has a poor formability since the coating layer has a high friction force, that is, a high friction coefficient since the coating layer has a large contact area with a die and is easily adhered to a de during its press forming due to the very rough and soft coating layer.
Therefore, the zinc- and zinc alloy-coated steel sheet itself may be cracked since a high friction load is applied to the steel sheet during the press forming of the steel sheet, which leads to the deteriorated productivity and workability in the manufacture of automobiles.
In this regard, a flash steel sheet may be used, for example, as the steel sheet whose formability is improved by decreasing its friction coefficient.
The flash steel sheet is a steel sheet obtained by thinly electroplating an alloy such as Fe—Zn, Fe—Mn or P—Fe alloy (a coating weight of 3,000 to 5,000 mg (milligrams)/m2 (square meter)) onto a zinc- and zinc alloy-coated steel sheet so that Fe or P can be present in a content of 90% by weight or more.
However, it is undesirable for automobile manufacture's to use the flash steel sheet in the aspect of the manufacturing cost due to its high manufacturing cost.
A phosphate-based, lubricant-coated steel sheet may also be used as the steel sheet whose formability is improved by decreasing its friction coefficient.
The lubricant-coated steel sheet is prepared by coating a zinc- and zinc alloy-coated steel sheet with a phosphate solution including a 2- or 3-component metal compound of nickel, manganese and zinc, all of which have excellent workability, and has an advantage that it has a superior formability and is inexpensive.
However, since phosphoric acid is weakly acidic, it is impossible to lower a free acid content of the phosphate solution below a predetermined extent. As a result, a large amount of free phosphoric acid remains in the coating layer.
The free phosphoric aid acts to degrade adhesion of adhesives, which are used to attach inner/outer plate of an automobile or the steel sheets in the hem flange of an automobile when an automobile body is assembled in the car manufacturing plant.
That is to say, the free phosphoric acid functions to degrade the adhesion between a surface of the steel sheet and the adhesives by weakening a binding force of the adhesives to the steel sheet, which leads to the degraded adhesion of the adhesives.
Therefore, the phosphate-based, lubricant-coated steel sheet (in the form of coated type), which is not washed after the coating, has good properties such as weldability, paintability and corrosion resistance including formability, but has a problem that the adhesion of the adhesives is very poor since a large amount of free phosphoric acid remains on the coating layer.
Korean Patent Application No. 2006-135682 filed by the present inventors proposes a steel sheet whose adhesion is improved while maintaining formability, weldability, paintability and corrosion resistance.
However, it was revealed that the adhesion of the steel sheet proposed in the Korean Patent Application No. 2006-135682 is rather degraded in the use of some adhesives.
When the assembly line of the automobile body is completed, it should also be subjected to alkaline degreasing and conversion coating processes, followed by an electrodepositing process. However, when a lubricant coating film is not completely removed from the steel sheets during the alkaline degreasing process, a grain shapes or size distribution in a conversion coating film is made irregular, which leads to the defects in a surface of the coating film, for example the formation of pinholes or craters during the electrodepositing process.
Furthermore, when a lubricant-coated steel sheet is manufactured using a roll water, a wettability of a coating solution may be degraded in respect to the steel sheet, and a lot of fine stripes may appear in a surface of the lubricant-coated steel sheet when the coating solution is dried slowly.
The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a coating composition for zinc- and zinc alloy-coated steel sheets capable of forming a coating film on a zinc- and zinc alloy-coated steel sheet, the coating film having excellent properties such as adhesive adhesion, alkaline coating removal property and lubricating property.
Also, it is another object of the present invention to provide a method for forming a coating film on a surface of a zinc- and zinc alloy-coated steel sheet, the coating film having excellent properties such as adhesive adhesion, alkaline coating removal property and lubricating property.
It is still another object of the present invention to provide a zinc, and zinc alloy-coated steel sheet including a coating film having excellent properties such as adhesive adhesion, alkaline coating removal property and lubricating property.
It is still another object of the present invention to provide a coating composition for zinc- and zinc alloy-coated steel sheets capable of forming a coating film on a zinc- and zinc alloy-coated steel sheet, the coating film having a fine surface appearance, as well as the excellent properties such as adhesive adhesion, alkaline coating removal property and lubricating property.
It is still another object of the present invention to provide a method for forming a mating film on a surface of a zinc- and zinc alloy-coated steel sheet, the coating film having a fine surface appearance, as well as the excellent properties such as adhesive adhesion, alkaline coating removal property and lubricating property.
It is yet another object of the present invention to provide a zinc- and zinc alloy-coated steel sheet inducing a coating film having a fine surface appearance, as well as the excellent properties such as adhesive adhesion, alkaline mating removal property and lubricating property.